Nowadays, polyesters are widely used in several products for human consumption, among which polyethylene terephthalate, better known as PET—saturated polyester from terephthalic acid and ethylene glycol—is one of the best known.
In recent years, PET consumption has especially soared since such compound is extensively employed in the manufacturing of containers for diverse liquid products, such as water and other bottled beverages. It is estimated that worldwide PET consumption adds up to more than 13 million tons distributed in three mayor markets, i.e. the textile, video tape and packing and container industries, the latter mainly comprising the manufacturing of bottles for beverages.
In connection thereinbefore, PET has been employed particularly in the manufacture of bottles for beverages due to its low weight, high strength, low permeability to gases and, above all, to the fact that PET has no deleterious effects on human health.
In spite of the advantages hereinabove concerning the use of PET, this material however brings about parallel environmental issues, since PET bottles occupy a large volume once they are disposed of and their degradation takes place quite slowly, given their significant resistance against atmospheric and biological agents. Thus, the PET is currently being classified as a pollutant agent.
As a result of such ecological issues and in tandem with economic concerns, the prior art has contemplated the recycling of PET and other polyesters by means of various techniques and processes, among which that known as “material recycling” is one of the simplest consisting of the collection, cleaning, grinding and granulation of the waste polymer, to incorporate it thereafter to the production of different items that need not comply with high quality and/or purity standards; hence, the field of application of this recycling technique is certainly narrow.
On the other hand, there exists what is known as “chemical recycling” (depolymerization), which comprises breaking the polyester chains. In this regard, an important number of chemical processes to depolymerize PET or other polyesters can be found in the prior art, such processes being classified in four major groups as follows: a) glycolysis, b) alcoholysis, c) hydrolysis and d) saponification.
Regarding glycolysis, it consists of degrading the polyester with diols such as ethylene glycol to temperature conditions from about 1800 to 250° C. When PET is decomposed via this process, the products obtained are mainly bis(hydroxyethyl)terephthalate (BHET) and ethylene glycol (EG) which is incorporated to the reacting media. As shown, one of the drawbacks in glycolysis is that high temperatures are required to perform it, which entails an important consumption of energy at an industrial scale.
One example of glycolysis decomposition may be found in the European Patent Serial No. 1,227,075 A1, which discloses a recovery method of dimethyl terephthalate (DMT) and ethylene glycol from polyester, notably PET. It is worth mentioning that the disclosed method contemplates the use of a depolymerization catalyst of polyesters in ethylene glycol, the reaction taking place at a temperature of from 175° to 190° C. and pressures ranging from 1 to 5 atm (0.1. to 0.5 MPa).
Regarding alcoholysis, polyester is degraded with alcohols, mainly methanol, wherein depolymerization occurs under temperature conditions of from 200° to 300° C. and pressures ranging from 2 to 300 atm; this presents a drawback because of the need of equipments that withstand such pressure. On the other hand, when PET is broken down, the main products obtained with such a process are dimethyl terephthalate (DMT) and ethylene glycol (EG).
A methanolysis process is disclosed in U.S. Pat. No. 5,051,528, wherein PET is dissolved in terephthalic acid and ethylene glycol oligomers, thereafter treated with methanol and obtaining in result DMT and ethylene glycol.
Concerning depolymerization by hydrolysis, it contemplates the rupture of the ester bond by means of OH ions. Likewise, hydrolysis takes into account the following variants:                i) Alkaline or basic hydrolysis, wherein an alkali is employed to break down the polyester, mainly NaOH, in an aqueous media and the reaction takes place under high temperatures and pressures; in other instances, the reaction media is ethylene glycol as well;        ii) Neutral hydrolysis, wherein the reaction takes place with the use of water at elevated temperatures; and        iii) Acid hydrolysis, wherein the polyester is broken down via the use of concentrated sulfuric acid.        
An example of alkaline hydrolysis is found on European patent Serial No. 0 973 715 B1, wherein PET is heated in an aqueous solution at temperatures ranging from 150° to 280° C. with a reactive agent selected from the group consisting of ammonia bicarbonates and alkaline metals.
Finally, in depolymerization with saponification, PET is molten down to treat it with strong bases such as potassium or sodium hydroxides at temperatures above 200° C.
Regarding the above, reference is made to the PET depolymerization process disclosed in U.S. Pat. No. 6,580,005 B1, which provides a process aimed at overcoming the disadvantages of traditional PET depolymerization processes. Particularly, such document discloses a method to recover terephthalic acid from ground PET waste, which method comprises (a) a decomposition reaction step, wherein ground PET waste undergoes a continuous decomposition reaction in ethylene glycol and in the presence of an alkali in an equimolar or excess ratio to PET, such that the salt of terephthalic acid and ethylene glycol can be afforded continuously; (b) a solid-liquid separation step, dissolution and removal of impurities, wherein ethylene glycol is separated from the terephthalic acid salt stemming from the decomposition reaction of terephthalic acid and ethylene glycol, and the terephthalic acid salt is dissolved in water, whereas insoluble impurities are removed; (c) a neutralization/crystallization step, wherein the solution of said terephthalic acid salt is neutralized with acid such that the terephthalic acid can be crystallized; (d) a washout/solid-liquid separation step, wherein the mass of terephthalic acid crystals undergoes a solid-liquid separation such that terephthalic acid crystals can be obtained and washed; and (e) a drying/grinding stage, wherein terephthalic acid crystals are washed, dried and ground.
From the process hereinabove it should be noted again that elevated temperatures are required in the decomposition reaction step, particularly in the ranger of from 130° and 180° C. and, as perceived in the examples included in the application, temperatures in the range of 180° C. to 190° C. must be achieved to favor the breaking of PET chains. Likewise, it is important to point out that prior to decomposition reaction a preheating stage is contemplated wherein ground PET is heated to temperatures ranging from 100° to 140° C., or a thermal degradation at temperatures ranging from 290° C. to 330° C. From the latter, it is noted that high temperatures must be achieved as well in these prior stages.
Another major drawback of the above process is the use of sodium carbonate as the alkali employed in the decomposition reaction, since this compound gives off carbon dioxide during the decomposition reaction, thereby increasing the reactor pressure; accordingly, this equipment must de designed to withstand such pressurization conditions.
In a nutshell, the chemical decomposition processes known in the prior art pose important disadvantages, particularly in the sense that such processes include a depolymerization reaction stage conducted at elevated temperatures and/or pressures, which in turn render them unattractive from an economic viewpoint due to their high energy consumption or because they require equipments that withstand high pressures. As a consequence, there is a major need of developing processes that, in addition to their efficacy, can also be appealing economic-wise.
As a result of the above, efforts have been made to overcome the obstacles posed by prior art chemical PET decomposition process through the development of a chemical recycling process of PET waste, the process including a depolymerization reaction stage (saponification) conducted at lower temperatures than those of prior art depolymerization reaction processes, such saponification stage being additionally carried out at atmospheric pressure or above. The products recovered under such process can be employed as starting material.